In recent years, telecommunications technologies became a field of the industry having one the greatest expansions ever. This is caused by the ever-increasing demand to broadcast larger amounts of data that require higher bandwidth of the telecommunication channel. One of the proposed wireless technologies that provides such a broad bandwidth is a modulation technique called the Orthogonal Frequency Division Multiplexing (OFDM) and which has been recently suggested for use in a digital video non-mobile broadcasting. OFDM appears to be a powerful means of providing a power efficient signaling for a large number of users on the same channel.
The basic idea of OFDM is to transmit blocks of data in parallel by employing a large number of orthogonal subcarriers.
Although OFDM transmission is an accurate modulation technique compared to other broadcasting techniques, there are two problems associated with it: one is frame synchronization and the other is channel equalization. In OFDM data is broken up into individually modulated sections called frames. These frames are strung together in a continuous manner and transmitted over an OFDM channel. It is of utmost importance for the receiver to know precisely where the frames begin a nd end, in order to recover the valuable data. Finding these beginnings and endings of the frames is what is known as frame synchronization. Various techniques of performing frame synchronization exists, but they usually involves adding data flags for marking the beginning or the ending of the frames. This is done at the expanse of adding portions of data that slow down the speed of transmitting valuable data.
Because OFDM channels sometimes distort the data they carry to the point that it is thoroughly unrecognizable, a scheme to correct any channel distortion is required. Such equalization techniques exist, but they often require the data correction to be done at the transmitter side so the state of the channel has to be known to the transmitter. Furthermore, these techniques do not apply to point to multipoint OFDM transmission since multiple paths are involved, each having its own different response.
It has to be noted that throughout the present application the term OFDM channel is used to designate the air path of the electromagnetic waves representing OFDM frames to be transmitted from a transmitting system to a receiving system. When a channel involves multiple receivers that capture the same transmitted signal, the present application refers to point-to-multipoint channel.